As emissions requirements for gas turbines have become more stringent, there has been a movement from conventional diffusion flame combustors to Dry Low NOx, (DLN) or Dry Low Emissions (DLE) or Lean Pre Mix (LPM) combustion systems. These DLN/DLE/LPM combustors use lean fuel air mixtures (equivalence ratio of 0.58 to 0.65) during fully premixed operation mode to reduce NOx and CO emissions. Because these combustors operate at such lean fuel/air (f/a) ratios, small changes in velocity fluctuations can result in large changes in mass flow and fuel air fluctuations.
The fluctuations can result in large variations in the rate of heat release and can result in high-pressure fluctuations in the combustion chamber. Interaction of the chamber acoustics, fuel/air fluctuation, vortex-flame interactions and unsteady heat release leads to a feed back loop mechanism resulting in dynamic pressure pulsations in the combustion system. This phenomenon of pressure fluctuations is called thermo acoustic or combustion dynamic instabilities. Combustion dynamics is a major concern in DLN/DLE/LPM combustors.
In the prior art, it has been suggested to mitigate combustion dynamics by providing a combustion liner cap assembly, and forming a second set of circumferentially spaced cooling holes through the cylindrical outer sleeve. Other prior art attempts to mitigate combustion dynamics include providing an external resonator, and active control by changing fuel flow.